1. Field of the Invention
This invention relates generally to location of emitters of electromagnetic radiation, and in one example, to Time Difference of Arrival (“TDOA”)/Frequency Difference of Arrival (“FDOA”) location of emitters of electromagnetic radiation.
2. Description of the Related Art
TDOA/FDOA techniques have been employed in the past to determine the location of emitters of electromagnetic radiation, such as radio frequency (“RF”) emissions. Using such techniques, the time difference (TDOA) in reception of a radio signal received at the sensing platforms of two spaced apart aircraft may be used to define a hyperboloid upon which the RF emitter of interest is located. Frequency difference (FDOA) in reception of the same radio signal at the sensing platforms of the two spaced apart aircraft may be generated by flying the spaced apart aircraft at different velocities and headings relative to the RF emitter, and may be used to further resolve the geo-location of the radio emitter. In this regard, assuming that the RF emitter of interest is located on the surface of the earth, the intersection of the TDOA hyperboloid, the FDOA surface, and the surface of the earth may be employed to determine a set of possible geo-locations for the RF emitter. However, only one of these possible geo-locations is real, the other possible geo-locations in the set are purely mathematical solutions that are not the real location of the radio emitter. To determine the correct geo-location requires additional information. In the past, this additional information has been obtained from a system that can produce a line of bearing, from a third aircraft that can measure an independent TDOA/FDOA set of data, or by repositioning the two aircraft and then measuring a second TDOA/FDOA set of data.
Where rapid emitter location determination is desired, past techniques have often employed three aircraft as multiple moving sensing platforms that receive RF signals from an RF emitter of unknown location. Using three aircraft allows the location of an RF emitter to be quickly determined based on a single measurement of time of arrival and frequency (Doppler) shift differences in RF signals received from the RF emitter at each of the three moving aircraft, without the need for repositioning of the three aircraft for subsequent measurement of RF signals received from the RF emitter. However, the necessity for utilizing a third aircraft increases operation costs and complexity.
Two aircraft have been employed as moving sensing platforms where sufficient time exists for measuring more than one set of TDOA/FDOA data from the RF emitter. This has been done by measuring a first set of TDOA/FDOA data at a first set of locations of the two aircraft followed by flying the two aircraft to a second set of locations and measuring a second set of TDOA/FDOA data at the second and new aircraft locations. The recomputed set of TDOA/FDOA data from the second set of aircraft locations may then be compared to the set of TDOA/FDOA data computed at the first set of aircraft locations to obtain a single overlapping answer for the geo-location of the emitter. However, where the RF signal emitter is only transmitting intermittently or only active for a short period of time, there may not be sufficient time to reposition the two aircraft for making subsequent TDOA/FDOA measurements.
Two aircraft may also be employed as moving sensing platforms in combination with additional equipment that is capable of generating a line of bearing, i.e., Inertial Navigation System (INS)/Global Positioning System (GPS) equipment, precision time reference equipment, and data link equipment. While such a technique allows the location of a RF signal emitter to be determined by measuring a set of TDOA/FDOA data at a single position of the two aircraft, it requires additional costs and complexity associated with the line of bearing equipment.